1. Field of the Invention
The present invention relates to an air emission control system for removing volatile organic compounds (VOCs) and other objectionable contaminants from an emitted gas stream which is generated from a water treatment means. Specifically the improved air emission control system of this invention relates to collecting, dehumidifying, bypassing, diluting, monitoring, purifying recycling and reusing the emitted gas stream, and also relates to feeding an inert gas , upon demand, for reducing oxygen content of said emitted gas stream, in turn, for preventing a gas purification contactor from being ignited and/or exploded due to oxidation of carbonaceous substances inside said gas purification contactor. A complete water treatment and gas emission control system is developed by applying the air emission control system to either single stage water treatment or multiple stage water treatment. The present invention which causes no air pollution is a low cost and highly efficient alternative to present site remediation and water treatment technologies.
About 70 percent of potable water in the U.S.A. is supplied by groundwater.
Groundwater contamination, which is a national major concern, is about 71 percent caused by industrial accidents (chemical spills, tank leaks, etc.), 16 percent caused by railroad or truck's chemical accidents, and 13 percent caused by leachates from lagoons or dumpsites.
The primary reasons for treating groundwater are: potable use (39 percent), clean-up of aquifer to prevent spread of contamination (48 percent), and industrial and commercial use (13 percent). In any case, the potentially hazardous VOCs must be removed. Timely clean-up of aquifer to prevent spread of contamination is extremely important because the damage can be beyond repair if the spread of contamination is too wide.
Toxic organic compounds commonly found in groundwater include, but will not be limited to, the following: carbon tetrachloride, chloroform, dibromo chloro propane, DDD, DDE, DDT, CIS-1,2 dichloro ethylene, dichloro pentadiene, diisopropyl ether, tertiary methyl butyl ether, diisopropyl methyl phosphonate, 1,3-dichloro propene, dichloro ethyl ether, dichloro isopropyl ether, benzene, acetone, ethyl acrylate, trichloro trifloro ethane, methylene chloride, phenol, orthochloro phenol, tetrachloro ethylene, trichloro ethylene, 1,1-trichloro ethane, vinylidiene chloride, toluene, xylene, EDB and others.
Conventional water treatment means for groundwater purification is an air stripping tower in which a groundwater containing toxic volatile organic compounds is introduced to the top of said air stripping tower forming a swarm of downward water droplets, while a bulk volume of air is introduced to the bottom of said air stripping tower forming an upward counter current air stream. In other words, water droplets enter the air phase (gas phase) for removing volatile organic compounds from said water droplets by air stripping action. An emitted gas stream containing toxic volatile organic compounds and other volatile contaminants is formed, and exits from the top of said air stripping tower. This emitted gas stream must be properly treated in order to prevent air pollution. An air stripping tower water effluent containing negligible concentration of volatile organic compounds is also formed . Although the efficiency of an air stripping tower for removing volatile organic compounds from a contaminated groundwater is high, disposal of its emitted gas stream containing toxic volatile organic compounds generally is a problem.
Conventional water treatment means for treating wastewater is a biological activated sludge process plant in which an aeration basin having a mixed liquor and suspended microorganisms is used for removing organics from wastewater by biochemical reactions in the presence of air bubbles, microorganisms and nutrients. Due to physical action of air bubbling, an emitted gas stream containing odorous, toxic, volatile organic compounds is also generated over the top of said aeration basin. Air emission control at activated sludge process plants is now an important environmental engineering task.
Recently several dissolved air flotation plants and dispersed air flotation plants are developed. These modern water treatment meanses utilize flotation technology for either water purification or wastewater treatment. Since air bubbles must be generated for removing volatile, surface-active, oily and/or suspended contaminants from a water stream, an emitted gas stream containing these contaminants is also formed over the top of a flotation plant, in turn, causing air pollution.
The method and apparatus of this invention have been developed specifically for air pollution control at air stripping towers, activated sludge process plants, dissolved air flotation plants, dispersed air flotation plants and other similar plants that generate gas bubbles and/or emitted gas streams.
In addition to treating the contaminated ground water, commercial, industrial or municipal wastewaters containing VOCs and other toxic volatile contaminants can all be efficiently treated by the process system of the present invention.
2. Description of the Prior Art
Conventional technologies for groundwater treatment include: air stripping without air emission control, granular activated carbon, chemical oxidation, and biological processes. Air stripping without air emission control is not acceptable in many states.
Granular activated carbon contactor is technically feasible for either water purification or air emission control, but may be economically unfeasible when it is used alone. Lawrence K. Wang et al (U.S. Pat. No. 5,122,165, Jun. 16, 1992) and Orest Hrycyk et al (U.S. Pat. No. 5,122,166, Jun. 16, 1992) have developed two physical-chemical processes for groundwater treatment both processes using a liquid phase granular activated carbon contactor for water purification and using a gas phase granular activated carbon contactor for air emission control. A biological process for groundwater treatment with air emission control has also been developed by Lawrence K. Wang et al (U.S. patent application Ser. No. 550,515, filed Jul. 10, 1990, now pending).
The above three inventions (U.S. Pat. Nos. 5,122,165 and 5,122,166; and U.S. patent application Ser. No. 550,515, filed Jul. 10, 1990) relate to efficient and cost-effective groundwater purification systems aiming at clean-up of aquifer to prevent spread of VOCs contamination in the environment. The purified groundwater can be discharged to a recharging well; while the purified gas is recycled to the system for gas stripping, thus eliminating a gas emission problem. The above three inventions consider the affordability, performance, governmental acceptance, air pollution prevention and simplified operation.
The present invention, however, relates to an improved air emission control system that can be used in conjunction with the above three inventions (U.S. Pat. Nos. 5,122,165 and 5,122,166; and U.S. patent application Ser. No. 550,515, filed Jul. 10, 1990) as well as with other prior art systems described by O'Brien and Fisher (Water Engineering & Management, May 1983), O'Brien and Stenzel (Public Works, December 1984), Stenzel and Gupta (Journal of the Air Pollution Control Association, December 1985), Krofta (U.S. Pat. Nos. 2,874,842, 3,182,799, 4,022,696, 4,184,967, 4,377,485, 4,626,345, and 4,931,175), Ying et al (U.S. Pat. Nos. 4,623,464, and 4,755,296), Copa et al (U.S. Pat. No. 4,810,386), Meidl (U.S. Pat. No. 4,857,198), Irvine et al (U.S. Pat. No. 5,126,050) and Wang et al (U.S. Pat. No. 5,069,783).
The prior art air pollution control systems for removing volatile organic compounds from an emitted gas stream include gas incineration and gas phase granular activated carbon adsorption. Gas incineration is efficient but extremely expensive. Granular activated carbon adsorption is affordable, but frequently causes combustion at carbon beds or even explosion due to interactions of carbon, volatile organic compounds, and oxygen during a rising temperature at carbon beds.
Prior art concerning treatment of a gas effluent from multistep liquid treatment systems has been reviewed. Carnahan et al merely treat a gas effluent in a reactor tank with chlorine, in accordance with their U.S. Pat. No. 4,919,814. Irvine et al suggests such gas effluent being treated by carbon adsorption followed by membrane separation in accordance with their U.S. Pat. No. 5,126,050 (Col. 11, lines 36-41). U.S. Pat. No. 4,894,162, awarded to Cournoyer et al in January 1990, suggests such gas effluent being treated by venturi dilution and collection in a tank where microorganism action purifies the gas. Anderson's U.S. Pat. No. 4,391,704 suggests venturi dilution, treatment with chlorine or ozone and adsorption. Meidl's U.S Pat. No. 4,857,198 suggests initial separate gas stripping followed by recycling of such gas effluent back to the treatment system containing biological solids and powdered adsorbent. A publication by Waltrip et al (Journal WPCF, Vol. 57, No. 10, 1985) suggests primarily treatment of such gas effluent in a scrubber.
The method and apparatus of this invention, however, relates to an air emission control system comprising a gas piping system, at least one gas mover, at least one gas dilution unit, a demister, a monitoring unit, at least one gas bypass unit, a gas purification contactor, a recycling unit, at least one gas sampling unit, and an inert gas source for preventing possible combustion or explosion to be occurred inside the gas purification contactor. Said gas purification contactor of this invention is packed with virgin granular activated carbon, virgin fibrous activated carbon, ion exchange resins, polymeric adsorbent, base treated activated carbon, aluminate treated activated carbon, base treated polymeric adsorbent, aluminate treated polymeric adsorbent, reticulated foam, fiberglass screen, fibrous activated carbon screen, coalescing filter screen, membrane filter media, or combinations thereof for removal of volatile contaminants from a gas effluent emitted from multistep liquid treatment systems.